LD5523KGL datasheet(9/17 Pages) LEADTREND

9 / 17 page

LD5523K

05/16/2018

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD5523K-DS-00 May 2018

Application Information

Operation Overview

The LD5523K is built in the multi-mode PWM controller, in

which operates a constant frequency to achieve the CCM

as heavy load. For demanding higher power efficiency

and power-saving in light load condition, the LD5523K

implements QR function to allow the valley switching and

accomplish zero voltage switching (ZVS). Under different

load conditions, LD5523K provides the different solutions

for achieving higher efficiency and performance.

Under Voltage Lockout (UVLO)

An UVLO comparator is implemented in it to detect the

voltage on the VCC pin. It would assure the supply

voltage enough to turn on the LD5523K PWM controllers

and further to drive the power MOSFET. As shown in Fig.

9, a hysteresis is built in to prevent the shutdown from the

voltage dip during startup. The turn-on and turn-off

threshold level are set at 16.5V and 7V, respectively.

Vcc

UVLO(on)

UVLO(off)

I(Vcc)

operating current (~ mA)

startup current

(~µA)

Fig. 9

Startup Current and Startup Circuit

The typical startup circuit to generate the LD5523K VCC

is shown in Fig. 10. During the startup transient, the VCC

is lower than the UVLO threshold thus there is no gate

pulse produced from LD5523K to drive power MOSFET.

Therefore, the current through R1 will provide the startup

current and to charge the capacitor C1. Whenever the

VCC voltage is high enough to turn on the LD5523K and

further to deliver the gate drive signal, the supply current

is provided from the auxiliary winding of the transformer.

Lower startup current requirement on the PWM controller

will help to increase the value of R1 and then reduce the

power consumption on R1. By using CMOS process and

the special circuit design, the maximum startup current of

LD5523K is only 1

A. If a higher resistance value of R1 is

chosen, it usually takes more time to start up. To select

the value of R1 and C1 carefully will optimize the power

consumption and startup time.

EMI

Filter

OUT

VCC

GND

LD5523K

input

Cbulk

Fig. 10

QR Mode Detection

The transformer will be demagnetized after the main

power MOSFET turns off. A quasi resonant signal will be

detected from auxiliary winding by FB pin through the

external resister.

As soon as the current of the secondary side diode is

down to zero during MOSFET-off period, the transformer

’s

core is demagnetized completely. VDS of MOSFET will

resonate in discontinuous current mode. The resonance

frequency (FQR) will be obtained as below.

)

(







Inductance of primary winding



Resonance equivalent parasitic capacitance

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